Aromatic dicyanates are a known class of thermosetting monomers which trimerize on heating and/or upon the action of a variety of catalysts to form branched polymers having s-triazine linkages (sometimes called polycyanurates or cross-linked cyanurates). Such aromatic dicyanates, including 2,2-bis(4-cyanatophenyl) propane [dicyanato derivative of bisphenol A or bisphenol A dicyanate or BCP], are commercially available in moderately pure forms as monomers or oligomers with molecular weights of about 2000. A representative process for preparing such monomers is described in U.S. Pat. No. 4,046,796 to Rottloff et al. by reaction of bisphenol A and similar bisphenols with a cyanogen halide in isopropanol with a tertiary amine catalyst.
It is known that aromatic dicyanates such as BCP can be heated or catalyzed by zinc chloride or other catalysts to set as a cross-linked polymer which is reasonably translucent, but quite yellow or brown in color. U.S. Pat. No. 4,026,913 to Tanigaichi et al. describes the polymerization of polycarbonates having hydroxy ends converted to dicyanato ends by cyanogen halide which are then polytrimerized with themselves or with monomeric aromatic dicyanates such as BCP to form cross-linked resins. Again, such compositions may be reasonably translucent but are normally quite yellow or brown in color.
U.S. Pat. No. 4,157,360 to Prevorsek and Chung describes thermoformable polymers which are an intimate mixture of a cross-linked polycyanurate network polymer and high molecular weight thermoplastic polymer such as a poly(ester-carbonate). It is described that intimate mixtures of aromatic dicyanate monomers such as BCP with thermoplastic polymers such as poly(ester-carbonates) can be set in fashions conventional for aromatic dicyanates to form interpenetrating network polymers of the thermoplastic finely dispersed in the cross-linked polycyanurate. While this patent describes such materials as having high impact strength, high Vicat softening temperature and good tensile properties, there is no description in the reference of the optical properties of the interpenetrating network polymer. Uses proposed for the interpenetrating network polymer include windscreens such as windshields, canopies, door windows and wire housings. In addition, it is described to deposit the interpenetrating network polymer as a coating on an article such as a wire, a conducting material, glass, poly(ester-carbonate) windshield and structural parts such as supporting beams. It is indicated that the cured composition will improve the abrasion and solvent resistance of the article.
For many applications of polycyanurates, and especially of polycyanurate networks with thermosetting polymers dispersed therein as described in U.S. Pat. No. 4,157,360, it is desirable that the polycyanurate not detract from the optical properties of either the thermosetting material dispersed therein or the substrate upon which the network is deposited. In this regard, conventional polycyanurates contribute excessive color that detracts from their use in windscreens and the like, even as a thin coating deposited on glass or on clear plastics such as polycarbonates and poly(estercarbonates).